Wow. This entire huge block of quotes above, minus <stuff>, is a
proposed definition of source? No, that wouldn't help at all; few
people will even want to read that much, much less figure out exactly
what it means, and figure out if it covers all bases. (I didn't read
it--sorry :)--because I believe any attempt at a strict definition of
source will fail, and I believe a page-long definition of *anything*
is doomed from the start.)
I don't really see how a strict definition helps, though. Back up a
little bit. If I create a complex, layered image in Photoshop or
Gimp, I probably prefer to edit it in those programs, using their
formats (eg. PSD). Exports to PNG are nearly uneditable by
comparison. I think it's clear that, for this particular case, the
source of the PNG is clearly the PSD. (This is a day to day,
real-life, practical case. I've requested PSD source from artists on
projects many times, in order to edit an image.)
In that case, should we expect the PSD to accompany the PNG (in the
source archive)?

If you take my <draft draft draft> definition above, yes. Unless the PNG
is layered exactly as the PSD, and can be interchanged (i.e. you can
convert from one to the other and back without loosing information).
It's there, in the strict definition. That is the only way it would help.

If we can't answer this, then there's no way we can hope to answer
similar questions for cases where "source" is less clear-cut (like
fonts).

The "comprehensive approach" I tried with my definition was exactly to
cut all the cases and try to cover all the bases :)

(To be clear, I'm not asking if DFSG#2 is interpreted to mean that,
or if the SC says that, but rather whether we should or not--is it in
the interests of the Project and its users. I suspect Manoj would
say "yes".)
(Also note that I'm purely talking about source requirements, not
licensing freedom. I believe it's extremely clear at this point that
we do expect such images to be licensed under DFSG-free terms, and I
fully agree with that.)